1. Field of the Invention
The present invention relates to a solar cell and a manufacturing method of the same. Here, the solar cell includes a p-side electrode and an n-side electrode on its back surface side that is opposite to its light receiving surface side.
2. Description of the Related Art
A solar cell is expected to be an alternative energy source because the solar cell can directly convert sun light, which is an unlimited source of clean energy, into electricity. Such a solar cell has been desired to be thinner because of the need to cut down the manufacturing cost along with expansion of the solar cell market, as well as the need to reduce the amount of silicon use along with shortage in the stock of silicon material. Additionally, the solar cell has been desired to be more efficient in light conversion as well as to be thinner.
Most crystalline silicon solar cells include a single crystalline silicon substrate, on one main surface of which a p type region is formed, and on the other main surface of which an n type region is formed. Collecting electrodes (such as extracting electrodes and line-shaped electrodes) are formed on the p type regions and then type regions, in order to collect current. It is important for the collecting electrodes to enlarge its cross-sectional area, in order to improve conductivity and thus to reduce resistance loss. In addition, it is also important to reduce so-called shadow loss, which is caused by the presence of these collecting electrodes blocking an incident light.
Accordingly, as a way to satisfy these demands, a so-called back contact solar cell has been proposed. In this solar cell, p-doped regions and n-doped regions are alternately disposed on its back surface side that is opposite to its light receiving side. The collecting electrodes are also formed on the back surface (see Japanese Patent Translation Publication No. 2006-523025).
Moreover, there has been proposed a way to satisfy the demands, in which a current is extracted from a back surface side of a substrate with the following structure. The substrate provided with a through-hole includes a doped layer formed on its light receiving surface side, the doped layer extending continuously on the light receiving surface side and on a periphery of the through-hole on the back surface side through the inner wall surface of the through-hole (see Japanese Unexamined Patent Application Publication No. Hei 2-51282).
Furthermore, as an example of a solar cell capable of achieving both thinness and high efficiency, there has been known a solar cell including a so-called HIT (Heterojunction with Intrinsic Thin-layer) structure. In the HIT structure, a substantially intrinsic amorphous silicon layer is sandwiched between a single-crystalline silicon substrate and an amorphous silicon layer, so that defects at the interface are reduced, thereby improving characteristics of a heterojunction at the interface. The solar cell with the HIT structure is capable of achieving both thinness and high efficiency, as compared with a crystalline silicon solar cell. Meanwhile, since such a solar cell with the HIT structure includes a front-to-back symmetric structure, a collecting electrode is formed also on a light receiving surface side. Accordingly, further high efficiency is expected to be achieved in such a solar cell with the HIT structure, by reducing shadow loss with the collecting electrode.
In the solar cell with the HIT structure, an i type amorphous silicon layer and a p type amorphous silicon layer are sequentially stacked, by the CVD method, on a light receiving surface of an n type single-crystalline silicon wafer. Additionally, in the solar cell with the HIT structure, an i type amorphous silicon layer and an n type amorphous silicon layer are sequentially stacked, by the CVD method, on a back surface side of the n type single-crystalline silicon wafer.